This invention relates to refiners of disc-type with opposed refining discs rotating relative to each other. The refining discs ale provided with refining elements, which between themselves form a refiner gap with a refiner zone for the working of fibrous material. The fibrous material preferably is lignocellulosic fiber material and the refiner is used for manufacturing, for example, reject pulp, recycled fiber pulp and mechanical pulps such as board pulp, thermomechanical pulp (TMP) and chemi-thermomechanical pulp (CTMP) as well as chemical pulps.
The invention, more precisely, relates to a refining element for use in a refiner of the aforesaid type.
A refining element is designed with a pattern of bars and intermediate grooves. The bars and grooves are formed in different ways, depending on the fibrous material to be worked and on the degree of working and thereby, in the case of lignocellulosic material, on the pulp quality desired. The bars, for example, can be continuous or discontinuous and arranged in different patterns.
The refiner gap is designed so that the fibrous material shall pass from the inside out, seen in radial direction. Farthest inward in the refiner gap the refining elements normally are designed to bring about a first disintegration of the material and to advance the material further outward in the refiner gap. A certain defibration, i.e. separation of the fibers of the lignocellulosic material, also takes place in the inner portion of the refiner gap where the distance between the refining surfaces is greatest. Thereafter the distance decreases outward for achieving the desired working of the fibrous material.
The working of the fibrous material is carried out substantially by the bars of the refining elements. Their design, thus, is of essential importance for the pulp quality. Other factors of influence on the pulp quality are, for example, the size of the refiner gap, the liquid contents in the fibrous material, the feed, temperature etc.
The bars have an upper surface with edges. At the working of the fibrous material the bars are worm, especially their edges, which thereby get round. In cases where one refiner disc is stationary, its bars most often get worn most, because the difference in speed between the fibrous material and stationary refiner disc is greater than the difference in speed between the fibrous material and rotating refiner discs.
The wear is caused above all by the fact that sand and other hard foreign particles follow along with the fibrous material into the refiner and, thus, into the refiner gap where they repeatedly come into contact with the bars of the refining elements.
The refiner discs normally have a rotation speed of up to 3000 revolutions per minute relative to each other, and the refiner gap normally has a size of about 0.2 to 2 mm. Foreign hard particles with a diameter greater than the refiner gap thereby can cause great damage on the refining elements, but also small particles subject the refining elements to wear.
When the leading bar edge due to wear is rounded off, the energy demand for manufacturing a desired pulp quality increases. The degree of working, and thereby the pulp quality, depend on the refiner gap, the size of which is controlled so that the desired pulp quality shall be obtained. With increased and uneven wear of the bar edges problems arise to maintain the desired pulp quality, which means that the refining elements must be exchanged.
The wear down of bars is an especially great problem at the manufacture of fiberboard pulp where the fibrous material often includes many impurities, for example stones and sand. The refining elements must be exchanged when they are worn, which implies a shutdown of the process. It is, therefore, desired to maintain the sharpness of the bar edges for as long as possible.
The present invention offers a solution of the aforesaid problems. According to the invention it is, thus, possible to use refining elements for a longer time without increased energy demand and with maintained pulp quality. By forming at least one step in the upper surfaces of the bars, at least two longitudinal edges located at different heights on the bars are formed. This means that initially the uppermost edge is active and subjected to wear. As the uppermost edge gradually gets worn, the edge work is taken over by the bar edge located nearest downward on the step. Hereby the service life for the refining element call be extended substantially.
The steps can extend, for examples along the entire length of the bars or be broken by small portions without step in the longitudinal direction of the bars. Each step can have along the bars a constant or varying depth into the upper surfaces of the bars. Tile steps can be formed on only one or on both sides of the bars. Preferably a single step is provided on a bar, but in certain cases two or more steps can be formed.
When steps are formed only on one side of the bars, the rotation direction of the refining discs carrying the refining elements cannot be changed. With regard to strength, however, this may still be suitable design.